1. Field of the Invention
The present invention relates to a recording head for carrying out printing by jetting ink drops for deposition at predetermined positions on recording media.
2. Description of the Prior Art
A method involving the use of a nozzle is typical of an ink-jet recording method which carries out printing by jetting liquid drops, particularly ink drops on a printing surface. Conventionally, the on-demand type and the continuous flow type are known as the nozzle-type recording method.
The on-demand-type method is used for printing in a manner that jets ink intermittently from a nozzle in accordance with recording information; it is primarily classified into the piezoelectric vibrator type and the thermal type. The piezoelectric vibrator type applies a pulse voltage to a piezoelectric element provided in an ink chamber to deform the piezoelectric element, thereby causing a change in ink liquid pressure within the ink chamber and jetting ink drops from the nozzle to record dots on recording sheets. The thermal type heats ink by a heating element provided within the ink chamber and jets ink drops from the nozzle by resulting bubbles to record dots on recording sheets.
On the other hand, the continuous flow type applies pressure to ink to continuously jet the ink from a nozzle, and at the same time applies vibration by means of a piezoelectric vibrator or the like to convert the jetted ink stream into liquid drops, and selects the liquid drops to apply electric charges and deflection for recording.
In any of these methods, the diameter of ink drops depends primarily on a nozzle diameter. Reducing the nozzle diameter poses problems, for example, that the nozzle is clogged by dust and dirt or a dry ink surface of the nozzle, or that the ink jet direction is changed due to residual ink deposited on the nozzle circumferential portions
On the other hand, several recording methods are proposed to perform printing by jetting ink drops on a printing surface instead of using a nozzle. For example, as disclosed in U.S. Pat. No. 4,308,547, there is a recording method by which a piezoelectric shell of sphere shape, curved in a concave shape, is disposed in ink and a voltage is applied to the piezoelectric shell via an electrode. In this method, longitudinal waves radiated into ink from the piezoelectric shell are converged at a point on the ink free surface and drops are jetted from the ink free surface.
As disclosed in Japanese Published Examined Patent Application No. Hei 6-45233, there is also a recording method by which a spherical concavity provided on a substrate such as glass is used as an acoustic lens and a vibrator consisting of a piezoelectric material and an electrode for applying voltage thereto is formed on the back of the substrate so that the vibrator is disposed in ink.
Further, there is disclosed in Japanese Published Unexamined Patent Application No. Hei 3-200199 a recording method by which a phase Fresnel lens of a thin film flat shape, as a more inexpensive lens capable of obtaining sharper focus, is mounted on the substrate in place of a concave lens.
According to the above mentioned method by which longitudinal waves are converged at an ink free surface so that drops are jetted from the ink free surface, the diameter of the drops is almost equal to the focusing diameter of the longitudinal waves, and the focusing diameter d is represented by d=F/f when the driving frequency of a vibrator is f and the F value of lens is F When the wavelength of longitudinal waves traveling through ink is represented by .lambda. and their traveling velocity by v, there is a relation v=f.multidot..lambda. between these and the driving frequency f of the vibrator.
Therefore, in the case of attempting to jet ink drops whose diameter (focusing diameter) d is as small as about 15 .mu.m, when the F value of lens is 1, since the speed v of longitudinal waves traveling through conventional water base ink with low viscosity is almost 1500 m/s, the driving frequency f of the vibrator must be set to a very high frequency such as about 100 MHz Since it is practically difficult to select a remarkably small value as the F value of the lens because of various problems, it follows that an attempt to select a smaller value as a drop diameter d generally requires that the vibrator be driven with a higher frequency.
As described above, the method by which longitudinal waves are converged on an ink free surface to jet drops from the ink free surface poses a problem of cost that driving means are generally expensive because a plurality of vibrators have to be driven at a frequency as high as about 100 MHz, and serious problems such as a change in a drop diameter caused by changed ink viscosity due to heating by absorption, and the disabled ink jetting capability due to ink that runs dry or solid within recording elements.
As another prior art, a novel recording method of on-demand type is disclosed in Japanese Published Unexamined Patent Application No. Hei 6-340070. This method brings a beam of a cantilever construction into resonance by bending vibration to cause sufficient amplitude to occur at the tip of the beam so that ink is jetted; it is a recording method which allows ink to be jetted at a relatively low driving frequency and a low voltage. However, since this method employs a mechanism which forms ink drops via a nozzle provided at a beam tip, the drop diameter depends on a nozzle diameter like a variety of recording methods described previously, so that this method has a problem that a reduced nozzle diameter would cause nozzle clogging and a change in an ink jet direction due to ink residuals deposited on the nozzle circumferential portions. Although the art disclosed by the Japanese Published Unexamined Patent Application No. Hei 6-340070, as one of technical problems to be solved, intends to reduce the possibility of nozzle clogging in comparison with conventional methods, it provides no fundamental solution because a drop diameter depends on a nozzle diameter.
As still another prior art, which is not the ink jet recording method, a method for transforming liquid into particles by energizing vibration energy is disclosed in Japanese Published Unexamined Patent Application No. Hei 3-154665. This method, which was made to apply to an atomizing apparatus, consists of a vibrator containing piezoelectric ceramics and a vibration section, secured to the vibrator, which makes bending vibration in the form of cantilever, and generates foggy liquid drops by radiation of supersonic waves with part of the vibration section immersed in liquid. However, the art used for the atomizing apparatus cannot apply to an ink jet recording apparatus which requires ink drops to be accurately deposited in specified positions of recording media, because a number of drops not controlled in terms of time and space are generated though minute drops can be generated without using a nozzle.
Thus, there occurs a problem that it is becoming difficult to accurately record images in increasing demand for higher definition in recent years on recording media such as paper, no matter what conventional recording methods or what arts in other fields such as atomizing apparatuses and the like are used.